Something I have been thinking about, I know on a boat while dealing with the listing and the work of sailing, getting in good cardio is not easy.

I was thinking about building a setup with some planetary gears and an alternator that could be pedaled and hooked up to the battery bank, so one one hand you charge your batteries, on the other hand you get a good workout.

I was on a yacht just the other day ... they had the alternator from a VW Polo(?) hooked up to an exercise bike... owner reckoned you were well knackered long before you produced any useful power

The big thing is getting the Alternator to turn fast enough. When you consider a vehicle has a crank pulley that is roughly 8" in diameter that turns approximately 800 rpm at idle, and the alternator has a 3" pulley.

That means at idle the alternator is turning at about 2,130 rpm, at 1500 rpm the alternator is turning 4,000 rpm.

I suspect the issue with the exercise bike your friend has setup is that it is not turning fast enough, you would need some serious gears to speed it up fast enough.

Our local science museum used to have a bike hooked up to a light bulb. Even when I was young, if I worked very hard I could only keep it lit all the way for a few seconds. In other words, unless you are a world class athlete, you are not getting much charge from it.

On the other hand, if you are going to exercise anyway, you may as well make that energy go for something. But since it is such a low return, there is unlikely to be a commercial version, so you'll have to invent it yourself.

Our local science museum used to have a bike hooked up to a light bulb. Even when I was young, if I worked very hard I could only keep it lit all the way for a few seconds. In other words, unless you are a world class athlete, you are not getting much charge from it.

On the other hand, if you are going to exercise anyway, you may as well make that energy go for something. But since it is such a low return, there is unlikely to be a commercial version, so you'll have to invent it yourself.

In other words, unless you are a world class athlete, you are not getting much charge from it.
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Actually, it is even worse than that. I used to work with world class athletes, building machines to test their power output in all sorts of sports, and it is quite depressing how little power even the very best of us can produce.

Actually, it is even worse than that. I used to work with world class athletes, building machines to test their power output in all sorts of sports, and it is quite depressing how little power even the very best of us can produce.

I built one years ago. There is a ton of info out there on the internet about pedal powered set ups and how much power you can actually expect to produce. There is also a big difference of what you can produce in 30 seconds as opposed to what you can maintain for 20 to 30 minutes. An automotive style alternator is not the most efficient generator, it requires a fairly high RPM.

My last attempt used an old 10 speed bike. The tire was removed from the rear wheel and it was used as a pulley for a long v belt that drove the generator.
The best generator I used was a variable speed fan motor from an air conditioning system. I built my own rectifiers and used a controller regulator designed for wind generators. A generator is really a motor running in "reverse". Put power to the motor it runs like a motor. Spin the shaft and power comes out of the connections.

All in all it was a fun project. It generated very little power, but I felt good that I was making electricity while I got a good half hour workout. I used it to power a small laptop or TV to watch while I exercised. If you stopped pedaling your TV or computer died.

Fun for the science of it, but I would follow someone's previous suggestion and just add more solar.

About 25Ah at 12V for a world class cyclist. A lot less than 10Ah for most of us.

From what I've read a person who is not an athlete, but is in fairly good condition, can maintain about 50-60 watts for 20-30 minutes. Of course, most anyone could produce more in short bursts, but if you're talking about getting any decent exercise out of it you have to maintain a constant rate for at least 20 minutes.

So, if you want to charge a 12 volt battery you have to produce at least 14 volts. 60 watts at 14 volts is about 4.3 amps. Meaning during a 30 minute workout you would put barely over 2Ah into your battery.

As mentioned, if you want the exercise for the sake of the exercise, it doesn't hurt to produce a little juice at the same time. But then it seems kind of silly (unless you have a REALLY big boat!) to waste the space on a stationary bicycle and generator setup for so little output.

As someone else said, buy a solar panel, and then take brisk walks on the beach.